The present invention relates to diode array target as used in electron tubes of the pick-up type. A particular diode target type of the mesa or pillar type is described in U.S. Pat. No. 3,821,092, and in copending application Ser. No. 462,918, filed Apr. 22, 1974, entitled "Deep Etch Metal Cap Silicon Diode Target" also owned by the assignee of the present invention. The diode target described in the above are mesa or pillar-type diode targets, with massive high surface area contact caps extending from the pillar diode on the read or output side of the target.
An oxide insulating layer is typically provided on the output surface of diode targets between the individual diodes to ensure that the scanning electrons land only on the contact caps and thus maintain the reverse bias potential across the diodes which is required to generate the output signal as a function of the input radiation.
The protruding metal contact caps extending from the diodes serve as electron beam acceptance pads and as protection against target deterioration. The target will have a dark current level increase with operating time as a result of soft X-ray damage to the semiconductive target. The size of the metal caps protects the area between the diodes to prevent electrons landing on the oxide layer and building up there as a distorting charge pattern. It was found that such distorting oxide charging could be eliminated by disposing a high resistivity surface leakage path on the oxide. However, the resistive layer imparted a poor lag characteristic to the target. The lag is the response time of the target to charging input radiation patterns and is largely a function of the capacitance of the target. The resistive layer diode target when initially operated does exhibit a degraded beam acceptance and longer lag characteristic.